Low-pass filter for a diplexer

ABSTRACT

Low-pass filter for a diplexer in an ADSL communications system comprising a low-pass filter ( 8 ) with chokes ( 9 ). Said chokes ( 9 ) have magnetic cores made of an alloy, especially a nanocristalline or amorphous alloy.

The invention relates to a low-pass filter for a diplexer to separatelow-frequency signals of analog communications systems fromhigh-frequency signals of digital communications systems with aplurality of longitudinal inductances connected in series and formed bymeans of magnetic cores.

The magnetic cores heretofore used were RM4, RM6, RM8 as well as otherferrite shell cores made of core materials such as, for example, N27 andN48. The required direct field bias capability and the necessarylinearity of the hysteresis loop were achieved by means of shear byslitting the ferrite core.

A drawback of shear is that it causes the effective core permeability tobe decreased to values of about 200. To reach the required magnetizinginductance, the volume of the ferrite core must therefore be very large,so that a low-pass filter made of ferrite cores requires a lot of space.A further disadvantage of the ferrite cores is the high number of turnsof the primary and secondary windings, which may cause ohmic losses andcapacitive interference effects.

EP 0 677 938 describes a diplexer comprising a low-pass filter and ahigh-pass filter. The low-pass filter has longitudinal inductances thatare connected in series and are formed by means of ferromagnetic cores.

EP 0 780 854 describes a current-compensated radio interferencesuppression choke that has a magnetic core made of a nanocrystallinealloy. The magnetic core has a permeability ranging from 10,000 to60,000, a saturation induction greater than 1 Tesla and a specificresistance greater than 90 μΩ. Based on the high permeability and thehigh saturation induction, the current-compensated radio interferencesuppression choke has a particularly small configuration. The propertiesof the nanocrystalline alloy are such that good interference suppressionis achieved despite the small configuration of the radio interferencesuppression choke.

Based on this prior art, it is the object of the invention to create alow-pass filter with low unit volume that meets the requirements forapplication in a diplexer with respect to direct field bias capabilityand filter quality.

This object is attained according to the invention in that the magneticcores are made of an alloy and the harmonics of the low-frequencysignals can be suppressed by the impedance of the longitudinalinductances, which increase with increasing frequency.

Amorphous and nanocrystalline alloys make it possible to producemagnetic cores with high saturation induction and a broad range ofpermeability values. Compared to conventional ferrite cores, magneticcores made of an amorphous or nanocrystalline alloy have therefore asubstantially smaller unit volume with comparable magnetic properties.It is difficult, however, to produce magnetic cores of amorphous ornanocrystalline alloys in such a way that the hysteresis loop has thelinearity required for application in broadband communications systems.A high measure of linearity is required, however, to ensure constantpermeability and thus constant inductance of the low-pass filter chokes.Otherwise dimensioning of the low-pass filter chokes is madesubstantially more difficult. It is generally required, however, toconnect several inductances in series to obtain the required filterquality. This series connection of the longitudinal inductancesincreases the interference attributable to non-linearities of thehysteresis loop. The harmonics produced by non-linearities of thehysteresis loops are attenuated, however, by the impedance of thelongitudinal inductances, which increases with increasing frequency. Itis thus possible, despite the lack of linearity of the hysteresis loopsof the employed magnetic cores, to build a low-pass filter from anamorphous or nanocrystalline alloy, which satisfies the linearityrequirements of the transfer characteristic. Due to the highersaturation induction and permeability of the nanocrystalline andamorphous materials compared to ferrites, smaller configurations for themagnetic core furthermore result.

The preferred alloys for use in low-pass filters are the subject of thedependent claims.

The invention will now be described in greater detail with reference tothe attached drawings in which:

FIG. 1 is an overview of the connection between a local switching centerand a subscriber side network termination;

FIG. 2 depicts a diplexer with a high-pass filter and a low-pass filter;

FIG. 3 shows a hysteresis loop with a high measure of linearity;

FIG. 4 is a diagram illustrating the dependence of permeability on thesuperimposed field strength;

FIG. 5 shows the frequency response of the real part and the imaginarypart of the permeability of low-pass filter chokes;

FIG. 6 shows the frequency response of the magnitude of the impedance ofthe low-pass filter chokes of FIG. 5; and

FIG. 7 shows the measured frequency response of the imaginary part ofthe permeability of a magnetic core with high surface roughness.

Since VDSL splitters have not yet been defined in every detail. ADSLsystem components are primarily described below. Based on currentknowledge, it has to be assumed that the requirements for the inductivecomponents for the VDSL low-pass system will largely correspond to thoseof the ADSL low-pass system.

As shown in FIG. 1, in the ADSL (Asymmetric Digital Subscriber Line)telecommunications system, an ADSL-capable digital local switchingcenter 1 is connected to an ADSL modem 2, the subscriber side networktermination, via a public two-wire line 3. POTS (Plain Old TelephoneSystem) or ISDN (Integrated Services Digital Network) connections mayrun synchronously via the two-wire line 3. The separation andtransmission of the low-frequency (POTS, ISDN) and the high-frequency(ADSL) components is carried out by diplexers 4 (POTS, ISDN splitters),which are located at the ends of the public two-wire line 3. Thehigh-frequency ADSL signals running over the public two-wire line 3 aredirected into an ADSL branch 5 by the diplexers 4, while thelow-frequency POTS and ISDN signals are respectively directed into aPOTS/ISDN branch 6 by the diplexers 4.

The diplexer 4 thus comprises a high-pass filter 7 and a low-pass filter8, which is formed of low-pass filter chokes 9 and capacitors 10.

It should be noted that different embodiments are possible for thelow-pass filter 8. For example, the low-pass filter 8 as shown in FIG. 2may be made exclusively of passive components or may contain activecomponents in addition. The low-pass filter chokes 9 presented here,however, may be used independently of the corresponding low-pass filter8.

A distinction is drawn depending on the transmission system between theDMT (discrete multitone) system and the CAP (carrieriess amplitude phasemodulation) system. The codings primarily affect the spectraldistribution of the exciting current of the high-pass branch 5 of thediplexer 4.

The maximum amplitude can be up to U_(ac)=30 V_(pp) in the ADSL systemand up to U_(ac)=15 V_(pp) in the POTS system. A direct current I_(dc)(POTS)<100 mA flows through the POTS branch and a direct current I_(dc)(ISDN)<80 mA through the ISDN branch. In POTS systems, there is also amaximum bell current I_(ac) the magnitude and frequency of which dependson the configuration of the bell generator. These currents result in aDC bias of the low-pass filter chokes 9, the magnitude of which isdecisively determined by the configuration of the low-pass filter choke9 and the material used for the magnetic cores.

I_(dc) (ISDN) or I_(dc) (POTS) and I_(ac) (POTS), in conjunction withU_(ac) modulation, may not cause the low-pass filter 8 to go either intosaturation or be modulated to such an extent that system-specificproperties defined in the relevant standards are no longer met.

The inductances used on both sides of the public two-wire line 3 mustmeet the following requirements:

a) minimum unit volume

b) suitable for the transmission, code systems

DMT

CAP

QAM/MQAM ((Multiple) Quadrature Amplitude Modulation)

c) magnetizing inductances<100 mH depending on filter configuration

d) DC superposition in power feed

0-100 mA in POTS+I_(ac), as a function of the configuration of the bellgenerator

0-80 mA in ISDN

e) loops in accordance with ANSI T1E1.413 and ETSI ETR 328

f) low core weight and SMD capability

g) toroidal core, therefore simpler safety requirements in accordancewith IEC 950.

These requirements may be met by small unslit toroidal tape cores madeof an amorphous, nearly magnetostriction-free cobalt-based alloy or apractically magnetostriction-free fine crystalline alloy.

The latter are usually referred to as “nanocrystalline alloys” and aredistinguished by an extremely fine grain with an average diameter ofless than 100 nm, which makes up more than 50% of the material volume.An important prerequisite is that the inductances have a high saturationinduction of B_(s)>0.6 T, preferably>0.9 T, and a very linear hysteresisloop with a saturation to remanence ratio B_(r)/B_(s)<0.2,preferably<0.08. Furthermore, the permeability must remain constant to avery large extent above 10 kHz in POTS splitters and far above 100 kHzin ISDN splitters. In addition it was found that low-pass filter chokes9 for POTS splitters may also be made of some amorphous iron-basedalloys. In either case, the saturation induction values, which are farabove 1 Tesla, are of great advantage. A list of all alloy systems thatwere considered and found suitable for use in diplexers is providedbelow.

The basic prerequisite for obtaining the, properties required for POTSsplitters and especially for ISDN splitters is a pronounced linearitycharacteristic of the hysteresis loops as depicted, for example, in FIG.3. Such lineal hysteresis loops may be obtained, for example, by theprocess steps described below:

The magnetically soft amorphous ribbon with a thickness d<30 μm,preferably<23 μm, produced by means of rapid solidification technologyfrom one of the alloys listed below is wound without tension on specialmachines to form the toroidal tape core in its final dimensions.

In the subsequent heat treatment to adjust the soft magnetic properties,a distinction must be drawn as to whether or not the core is made of analloy that is suitable for establishing a nanocrystalline structure.

Toroidal tape cores made of alloys suitable for nanocrystallization aresubjected to a crystallization heat treatment to adjust thenanocrystalline structure, which ranges from 450° C. to 680° C.depending on the alloy composition. Typical holding times range from 4minutes to 8 hours. Depending on the alloy, this crystallization heattreatment under vacuum must be carried out in passive or reducingprotective gas. In all cases, material-specific purity conditions mustbe taken into account, which must be brought about by correspondingagents on a case by case basis, e.g., by element-specific absorbers orgetter materials. Through an exactly adjusted combination of temperatureand time, use is made of the fact that in the alloy systems specifiedbelow, the magnetostriction contributions of the fine-crystalline grainand the amorphous residual phase are equalized and the required freedomfrom magnetostriction (|λ_(s)|<2 ppm, preferably even|λ_(S)|<0.2 ppm) iscreated. Depending on the alloy and the configuration of the low-passfilter choke 9, tempering is carried out either fieldless or in amagnetic field longitudinal to the direction of the wound ribbon(“direct-axis field”) or perpendicular thereto (“quadrature-axisfield”). In certain cases, a combination of two or even three ofthese-magnetic field constellations may become necessary. Particularlyflat and linear loops can be obtained if the toroidal tape cores areprecisely stacked end to end, so that the stack height is at least 10times, preferably at least 20 times the core outside diameter, and astrong quadrature field is applied already during the describedcrystallization heat treatment. The hysteresis loop becomes all theflatter, the higher the quadrature field temperature is set, with allupper temperature limit being defined at the point where alloy-specificCurie temperatures are exceeded and non-magnetic phases are created.

The magnetic properties, i.e., the linearity and the slope of thehysteresis loop, can be varied over a wide range—if necessary—by anadditional heat treatment in a magnetic field that is parallel to therotational axis of symmetry of the toroidal tape core—i.e., normal tothe tape direction. Depending on the alloy and the permeability level tobe adjusted, temperatures of between 350° C. and 680° C. are required.Based on the kinetics of the atomic reorientation processes, theresultant permeability values are all the lower the higher thequadrature field temperature is. Typical characteristics of differentnanocrystalline magnetic alloys used according to the invention areshown in FIG. 4. This magnetic field heat treatment may be eithercombined directly with crystallization annealing or carried outseparately. For the annealing atmosphere the same conditions apply asthose for annealing to adjust the nanocrystalline structure.

In toroidal tape cores made of amorphous materials the magneticproperties, i.e., course and slope of the linear flat hysteresis loop,are adjusted by a heat treatment in a magnetic field running parallel tothe rotational axis of symmetry of the toroidal tape core—i.e., normalto the tape direction. Through proper control of the heat treatment itis possible to make use of the fact that the value of the saturationmagnetostriction during the heat treatment changes in positive directionby an amount that depends on the alloy's composition until it meets therange |λ_(s)|<2 ppm, preferably even |λ_(s)|<0.1 ppm. As shown in Table2, this can be achieved even if the amount of λ_(S) in the as quenchedstate of the tape is clearly above this value. Important is thatdepending on the employed alloy, the toroidal tape core is purged withair or a reducing (e.g., NH₃, H₂, CO) or a passive protective gas (e.g.;He, Ne, Ar, N₂, CO₂) so that neither oxidation nor other reactions canoccur on the tape surfaces. There may also be no solid state physicalreactions in the interior of the material through inward diffusingprotective gas.

Typically, the toroidal tape cores for the low-pass filter 8, dependingon the employed alloy composition, are heated to temperatures of between220° C. and 420° C. at a rate of 0.1 to 10 K/min as a magnetic field isapplied, kept in this magnetic field and within this temperature rangefor 0.5 to 48 hours, and subsequently cooled again to room temperatureat a rate of 0.1-5 K/min. Due to the temperature dependence of themagnetic moments, the resultant loops in the amorphous alloys usedaccording to the invention are all the flatter and more linear, thelower the quadrature field temperatures are. Particularly flat andlinear loops may be obtained if the toroidal tape cores are stackedend-to-end in such a way that the height of the stack is at least tentimes, preferably at least 20 times the outside diameter of the core.Typical characteristics are shown in FIG. 4.

On a case by case basis, a temperature plateau in the quadrature fieldmay be dispensed with and the preferred magnetic orientation produced bycooling the toroidal tape cores in the quadrature field. Thepermeability level is then adjusted via the cooling rate below the Curietemperature of the magnetic material.

Following the heat treatment, the toroidal tape cores are surfacepassivated, coated, fluidized-bed sintered or encapsulated in a trough,provided with windings and, if required, bonded or potted in thecomponent housing. This process is independent of whether the toroidaltape core is made of an amorphous or a nanocrystalline material. Due tothe brittleness, however, mechanical handling of the temperednanocrystalline toroidal tape cores must be carried out very carefully.

A further production option is to subject the tape first to quadraturefield tempering in a continuous process and then to wind it into thetoroidal tape core. The further sequence corresponds to that describedabove.

The toroidal tape cores produced according to this process meet thefollowing requirements:

The magnetizing inductance of the wound toroidal tape core, depending onthe layout of the low-pass filter choke 9, ranges from 1 to 100 mH, inspecial configurations of low-pass filters 8 (e.g., ellipticalcharacteristic) the magnetizing inductance is also<1 mH.

In addition, the magnetizing inductance also meets this value with DCsuperposition and maximum alternating modulation as well as thefrequencies defined by standard.

The linearity error of the hysteresis loop of the toroidal tape core isso low that the following holds:

μ (4/5*B _(s))/μ (1/100*B _(s))>0.7, preferably>0.9.

The bit error rates that can be obtained in typical user circuitsconform to the standards (FTZ 1TR220 and ETSI ETR 80).

Particularly when using an amorphous alloy of the compositionCo_(55.6)Fe_(6.1)Mn_(1.1)Si_(4.3)B_(16.2) Ni_(16.2), the typical minimumcore dimensions shown in Table 1 result after balanced quadrature fieldtempering at 280° C.-360° C. for predefined values of magnetizinginductance and DC loadability.

TABLE 1 I_(de), L_(magnet) max Core Dimensions Core Weight [rnH] [mA][mm³] [g]  6.2 100  9.2 × 6.5 × 4.5 0.96  7.9 100  9.8 × 6.5 × 4.5 1.22 9.2 100  9.8 × 6.5 × 4.5 1.22 12.1 100 11.3 × 8.0 × 5.5 1.81 16.9 10012.3 × 8.0 × 5.5 2.48 27.6 100 14.3 × 8.0 × 5.5 4.00

Similar core dimensions also result when using the other alloys listedbelow, which are employed for specific applications.

When dimensioning the low-pass filter chokes 9, a number of relationsmust be taken into account.

For the inductance of the transformer, the following relation holds:

L=N ²μ_(o)μ_(r) A _(fe/) L _(fe)  (1)

N=number of turns

μ_(o)=universal permeability constant

μ_(r)=permeability of the material

A_(fe)=iron cross section of the toroidal tape core

L_(fe)=iron path length of the: toroidal tape core.

From equation (1) it is evident that the required inductance call bereached with the minimum unit volume only if the number of turns,permeability, core cross section and iron path length arc tuned relativeto one another. The permeability of the core material, in addition tothe favorable toroidal geometry, is the decisive parameter for the mostcompact possible dimensioning of the low-pass filter choke 9. Dependingon which of the alloys listed below is used and how the associated heattreatment is conducted, a permeability range between 500 and 120,000 canbe covered in a defined manner. In the low-pass filter chokes 9 suitablefor use in ADSL systems, the permeability range below 20,000 ispreferably used, which brings a high degree of flexibility with respectto dimensioning the inductances. The low-pass filters 8 built with thesetoroidal tape cores have a strong volume advantage compared to slitferrite cores (μ=100-400) as well as great electrotechnical advantagesdue to the low number of turns of the winding.

The selection of the core material for the inductances of the low-passfilter 8 is fundamentally limited in that the inductance may not bemagnetized to near saturation by I_(dc) (ISDN) or I_(dc) (POTS) andI_(ac) (POTS).

The direct currents superimposed on the: signal modulation lead to adirect field bias

H _(dc) =I _(dc) N/L _(fe)  (2)

below which the permeability may drop only slightly. For this reason,the material is evaluated by means of μ(H_(dc)) characteristics, such asthose shown by way of example in FIG. 4 for different amorphous andnanocrystalline material core combinations developed for the low-passfilter chokes 9.

Since according to FIG. 4 the usable constant working range of the μ(H_(dc)) characteristic depends on the magnitude of the anisotropy fieldstrength

H _(a) =B _(s)/(μ_(o)*μ_(r))  (4)

the alloy composition in combination with the quadrature field heattreatment must be defined in such a way that on the one hand saturationinduction is as high as possible while on the other hand permeability isas low as possible. However, since according to Equation 1 aparticularly low permeability must be compensated by increasing thenumber of turns N, which in turn causes the direct field bias H_(dc)defined in Equation 2 to increase, a compromise must be found betweenhigh anisotropy field strength and adequately high permeability whenselecting the alloy and heat treatment for low-pass inductances.

When dimensioning the low-pass filter chokes 9, care must furthermore betaken that the magnitude of the impedance of the low-pass filter chokes9 increases with increasing frequency, since this suppresses harmonicsproduced by non-linearities in the hysteresis loop of the toroidal tapecore.

This will be explained in greater detail below by means of FIGS. 5 to 7.

In the serial equivalent circuit diagram,.the low-pass filter choke 9 isrepresented by an ohmic resistance connected in series and an idealinductance. The magnitude of the ohmic resistance is determined by theimaginary part of the permeability. The real part of the permeability onthe other hand defines the magnitude of the ideal inductance.

FIG. 5 shows the typical dependence of the real part μ_(s)′ and theimaginary part μ_(s)″ of frequency f. The figure shows the course fortwo exemplary embodiments with different initial permeability. Curve 11represents the course of the real part μ_(s)′ of an exemplary embodimentwith an initial permeability of 37.000. The dashed curve 12 indicatesthe course of the imaginary part μ_(s)″ of this embodiment. Analogously,the solid curve 13 and the dashed line 14 respectively represent thereal part μ_(s)′ and the imaginary part μ_(s)″ of an exemplaryembodiment with an initial permeability of 800.

Based on FIG. 5 it may be seen that the imaginary parts μ_(s)″ of thepermeability have a maximum at an eddy current critical frequency f_(g).Within the range of the same frequency, the real part μ_(s)′ of thepermeability begins to drop.

FIG. 6 shows the resultant course of the magnitude of impedance Z. Thetwo embodiments are designed in such a way that they have the sameinductance at least at low frequencies. Accordingly, the toroidal tapecore with an initial permeability of 800 is wound with 48 turns, whilethe toroidal tape core with an initial permeability of 37,000 is woundwith 8 turns. Since both low-pass filter chokes 9 have the sameinductance at low frequencies, a curve 15 characteristic for theembodiment with the high initial permeability differs only slightly forlow frequencies from a curve 16 characteristic for the embodiment withhigh initial permeability. At the eddy current critical frequency f_(g),however, curve 15 begins to flatten out somewhat and then finally dropsdue to the capacity of the winding above a maximum. The same applies tocurve 16. When it approaches the critical frequency f_(g), curve 16begins to flatten out somewhat before it finally drops on the other sideof a maximum due to the large capacity of the winding formed by 48turns.

The frequency response of the impedance.also affects the filter qualityof the low-pass filter 8. The harmonics of the low frequency signalproduced by the non-linearities of the hysteresis loop can be imaginedto be produced by an interfering voltage source that is connected to aterminating resistance at which the output signal is picked off. Theinternal resistance of the interfering voltage source is then determinedby the impedance of the low-pass filter choke 9. Since at highfrequencies the impedance of the low-pass filter choke 9 stronglyincreases, the interfering voltage produced by the interfering voltagesource drops primarily at the internal resistance of the interferingvoltage source and not at the terminating resistance. In this manner,the harmonics of the low frequency signal produced by non-linearities ofthe hysteresis loop are effectively suppressed. This presumes, however,a sufficiently strong impedance increase at high frequencies. Thisincrease in the impedance should occur at least up to the fifthharmonic, preferably up to the eleventh harmonic. FIG. 6 clearly showsthat in this connection it is advantageous in principle if the initialpermeabilities are selected low, so that based on the high eddy currentcritical frequency the impedance strongly increases even at highfrequencies. In contrast to exemplary embodiments in which the initialpermeability is selected high, this has the additional advantage thatthe toroidal tape core is modulated very little due to the lowpermeability. Since in this case only a small part of the hysteresisloop is descending, non-linearities that occur with large modulation ofthe toroidal tape core are of less consequence.

It should be noted that the relations shown in FIGS. 5 and 6 are trueonly if other dissipative effects aside from the generation of eddycurrents are to be ignored. For example, if during production of thetoroidal tape cores, heat treatment takes place with inadequate purgingwith protective gas, SiO₂ layers may grow on the surface of the magnetictapes, which increase the domain walls in the magnetic tape. Theaccompanying roughnesses of the tape surface furthermore represent seedsfor wall shifting processes. Conversely, demagnetized fields are formeddue to the surface roughness, which prevents rapid demagnetization.Furthermore, migration of the domain walls induces strong eddy currentsthat counteract the shifting of the domain walls. Overall, the shiftingof the domain walls in the alternating field represents a furtherdissipative process, which at frequencies far below the eddy currentfrequency may lead to a further maximum of the imaginary part μ_(s)″ ofthe permeability. FIG. 7 shows the frequency response of the imaginarypart μ_(s)″ of the permeability as a function of the strength of theapplied external magnetic field. Since the above-described effects aremore or less pronounced as a function of the strength of the appliedexternal magnetic field, the frequency response of the imaginary partμ_(s)″ of the permeability depends not only on the frequency but also onthe strength of the applied external magnetic field. It is thereforeessential to wind the toroidal tape cores as stress-free as possible andto carry out the heat treatment in a flowing protective gas atmosphere.

If the above conditions are met, the alloy system described below makesit possible to produce low-pass filters that have all the properties inconformance with standards, with particularly linear hysteresis loopsand small configurations.

Alloy System 1:

An alloy system suitable for POTS low-pass filters has the compositionFe_(a)M_(b)Si_(x)B_(y)R_(z), where M stands for one or more elementsfrom among the group Co, Ni and R for one or more elements from amongthe group C, V, Nb, Mn, Ti, Cr, Mo, W, and a+b+x+y+z=100%, where

Fe a = 61-82 at % Co/Ni b = 0-20 at % Si x = 0.5-19 at % B y = 7-23 at %R z = 0-3 at %

where 70<a+b<86, preferably 73<a+b<85 at % and 14<x+y+z<30 at %.

Alloys of this system remain amorphous after heat treatment, but due totheir relatively large saturation magnetostriction, which may be+20 ppm,their linearity characteristics are not quite as pronounced as those ofthe following Co-based alloys. They are therefore primarily suitable forPOTS low-pass filters.

Alloy System 2:

A further suitable alloy system has the composition Co_(a)(Fe_(l-x)Mn_(x))_(b)Ni_(d) M_(e)Si_(x)B_(y)C_(z), where M is one or moreelements from among the group Nb, Mo, Ta, Cr, W, Ge and/or P anda+b+c+d+e+x+y+z=100, where

Co a = 40-82 at % preferably a > 50 at % Mn/Fe x = 0-1 preferably x <0.5 Fe + Mn b = 3-10 at % Ni: d = 0-30 at % preferably d < 20 at % M: e= 0-5 at % preferably e < 3 at % Si: x = 0-15 at % preferably x > 1 at %B y = 8-26 at % preferably 8-20 at % C z = 0-3 at %

15<e+x+y+z<30 (preferably 18<e+x+y+z<25).

Alloys of this system remain amorphous after heat treatment, meet therequirement |λ_(s)|<0.1 ppm particularly well and, due to their linearloop form and the extremely good frequency response, are particularlywell suited for both ISDN and POTS low-pass filters.

Alloy System 3:

A third suitable alloy system has the compositionFe_(x)Cu_(y)M_(z)Si_(v)B_(w), where M is an element from among the groupNb, W, Ta, Zr, Hf, Ti, Mo, or a combination thereof, and x+y+z+v+w=100%,where:

Fe x = 100% − y − z − v − w Cu y = 0.5-2 at % preferably 1 at % M z =1-5 at % preferably 2-3 at % Si v = 6.5-18 at % preferably 14-17 at % Bw = 5-14 at %

where v+w>18 at %, preferably v+w=22-24 at %.

Alloys of this system, due to their linear loop form and the very goodfrequency response, are well suited for both ISDN and POTS low-passfilters.

Alloy System 4:

A further suitable alloy system has the compositionFe_(x)Zr_(y)Nb_(z)B_(v)Cu_(w), where x+y+z+v+w=100 at %, where:

Fe 100 = 100 at % − y − z − v − w preferably 83 . . . 86 at % Zr y = 2-5at % preferably 3-4 at % Nb z = 2-5 at % B v = 5-9 at % Cu w = 0.5-1.5at % preferably 1 at %

where y+z>5 at %, preferably 7 at % and y+z+v>11, preferably 12-16 at %.

Alloys of this system, due to their lineal loop form and their very goodfrequency response, are well suited for both ISDN and POTS low-passfilters.

Alloy System 5:

A further alloy system has the composition Fe_(x)M_(y)B_(z)Cu_(w), whereM is an element from among the group Zr, Hf, Nb and x+y+z+w=100 at %,where:

Fe x = 100 at % − y − z − w preferably 83-90 at % M y = 6-8 at %preferably 7 at % B z = 3-9 at % Cu w = 0-1.5 at %

Alloys of this system, due to their linear loop form and their very goodfrequency response, are well suited for both ISDN and POTS low-passfilters.

Alloy System 6:

A sixth alloy system suitable for low-pass filters in diplexers has thecomposition (Fe_(0.98)C_(0.02))_(90−x)Zr₇B_(2+x)Cu₁ where x=0-3,preferably x=0, wherein Co may be replaced by Ni with correspondingadjustment of the remaining alloy components.

Alloys of this system, due to their linear loop form and their very goodfrequency response, are well suited for both ISDN and POTS low-passfilters.

After heat treatment, the alloy systems 3 to 6 receive a finecrystalline structure with grain diameters of less than 100 nm. Thesegrains are surrounded by an amorphous phase which, however, makes upless than 50% of the material volume.

Alloy System 7:

POTS low-pass filters may furthermore be produced from metal tape coresof crystalline rolled ribbons. To achieve the necessary frequencycharacteristics of the permeability, the ribbon thickness must be lessthan 50 pm, preferably less than 30 pm. A functional alloy system wasfound to be:

Ni _(x) Fe _(y) M _(z)

where M is one or more elements from among the group Nb, Mo, Mn, Si, Al,Ti, Cr and x+y+z=100 at % where

Ni x = 35-68 at % preferably 50-65 at % Fe y = 100 at % − x − z M z =0-9 at % preferably 0.5-3 at %

In the production of the metal tape cores, depending on the alloycomposition, a further tempering treatment in the range of 450° C.-650°C. may be necessary in addition to recrystallization annealing at800-1250° C. To meet the linearity characteristics conforming tostandards, both treatments must be carried out in a protective reducinggas. To achieve a flat linear hysteresis loop, the preferred magneticdirection is adjusted, depending on the Ni content, either by aquadrature field heat treatment or by cold forming, e.g., by anadditional rolling step.

Aside from the alloys of Groups 1 to 6, which are useful for POTSlow-pass filters, additional characteristics of the preferred alloysystems 2 to 6 are:

extremely linear hysteresis loop with at least H=600 mA/cm or higher,

Magnitude of saturation magnetostriction |λ_(s)|<2 ppm, preferably<0.1ppm after heat treatment. In the cobalt-based amorphous materials, thesaturation magnetostriction may be adjusted by correspondinglyfine-tuning the Fe and Mn content. In the nanocrystalline alloys, thesaturation induction may be adjusted via the size of the finecrystalline grain, which can be accomplished by a specific tuning of theheat treatment and the metalloid content.

Saturation induction of 0.7 T-1.7 T; the saturation induction can befine-tuned through the selection of the Ni, Co, M, Si, B and C content.

In the above alloy systems the less than/greater than symbols includethe limits; all at % indicated arc approximate values.

Particularly the alloy examples listed in Table 2 meet and satisfy theaforementioned requirements and alloy ranges after the described heattreatment has been carried out. Based on their combination of aparticularly low constant permeability and a saturation induction ofnearly 1 Tesla, the examples of amorphous cobalt-based alloys listed inTable 2 exhibit particularly high anisotropy field strengths, in somecases more than 8 A/m. In contrast, the fine and nanocrystalline alloysalso listed in Table 2 are distinguished by particularly high saturationinduction values up to 1.7 Tesla. They permit comparatively highpermeability values, which according to the invention provides furtheradvantages compared to ferrite transformers with respect to overalldimensions and winding.

TABLE 2 Saturation Saturation Anisotropy Magnetostriction λ_(s) AlloyComposition Induction Field Strength as heat- [at %] Structure [T] H_(a)[A/cm] quenched treated Co_(71.7)Fe_(1.1)Mo₁Mn₄Si_(13.2)B₉ amorphous0.82 1.5 −12 * 10⁻⁸ −3.5 * 10⁻⁸ Co_(72.5)Fe_(1.5)Mo_(0.2)Mn₄Si_(4.8)B₁₇amorphous 1.0  3.5 −12 * 10⁻⁸ −4.1 * 10⁻⁸ Co_(72.8)Fe_(4.7)Si_(5.5)B₁₇amorphous 0.99 4.8 −32 * 10⁻⁸ −1.6 * 10⁻⁸Co_(55.6)Fe_(6.1)Mn_(1.1)Si_(4.3)B_(16.2)Ni_(16.5) amorphous 0.93 8.0−110 * 10⁻⁸  +4.2 * 10⁻⁸ Fe_(73.5)Cu₁Nb₃Si_(15.5)B₇ nanocryst. 1.21 0.7+24 * 10⁻⁶ +1.6 * 10⁻⁷ (Fe_(0.98)Co_(0.02))₉₀Zr₇B₂Cu₁ nanocryst. 1.701.7 — −1.0 * 10⁻⁷ Fe₈₄Zr_(3.5)Nb_(3.5)B₈Cu₁ nanocryst. 1.53 0.8  +3 *10⁻⁶ +1.5 * 10⁻⁷ Fe₈₄Nb₇B₉ nanocryst. 1.5  1.2 — +1.0 * 10⁻⁷

What is claimed is:
 1. Low-pass filter for a diplexer for separating lowfrequency signals of analog communications systems from high frequencysignals of digital communications systems, the low-pass filtercomprising a plurality of longitudinal inductances connected in series,such longitudinal inductances (i) comprising magnetic cores made of anamorphous or nanocrystalline alloy, (ii) having impedance whichincreases with increasing frequency and suppresses harmonics of thelow-frequency signals, and (iii) having a saturation inductionB_(s)>0.6T, a saturation to remanence ratio B_(r)/B_(s)<0.2 and apermeability that is constant to a very large extent up to above 10 kHz.2. Low-pass filter as claimed in claim 1 characterized in that the alloyhas the composition Fe_(a)M_(b)Si_(x)B_(y)R_(z), where M is one or moreelements from among the group Co, Ni and R is one or more elements fromamong the group C, V, Nb, Mn, Ti, Cr, Mo, W and a+b+x+y+z=100%, where:Fe a = 61-82 at −% Co/Ni b = 0-20 at −% Si x = 0.5-19 at −% B y = 7-23at −% R z = 0-3 at −%

 where 70<a+b<86 at-% and 14<x+y+z<30 at-%.
 3. Low-pass filter asclaimed in claim 2 characterized in that the relation 73<a+b<85 at-% istrue for a and b.
 4. Low-pass filter as claimed in claim 1 characterizedin that the alloy has the compositionCo_(a)(Fe_(1−x)Mn_(x))_(b)(Ni_(d)M_(e)Si_(x)B_(y)C_(z), the alloy hasthe composition Co_(a)(Fe_(1−x)Mn_(x))_(b)Ni_(d)M_(e)Si_(x)B_(y)C_(z),where M is one or more elements from among the group Nb, Mo, Ta, Cr, W,Ge and P and a+b+c+d+e+x+y+z=100, where: Co a = 40-82 at % Mn/Fe x = 0-1Fe + Mn b = 3-10 at % Ni d = 0-30 at % M e = 0-5 at % Si x = 0-15 at % By = 8-26 at % C z = 0-3 at %

 where 15<e+x+y+z<30.
 5. Low-pass filter as claimed in claim 4,characterized in that the following relations hold: C a = 50-82 at %Mn/Fe x = 0-0.5 Fe + Mn b = 3-10 at % Ni d = 0-20 at % M e = 0-3 at % Six = 1-15 at % B v = 8-20 at % C z = 0-3 at %

 where 18<e+x+y+z<25.
 6. Low-pass filter as claimed in claim 1,characterized in that the alloy has the compositionFe_(x)Cu_(y)M_(z)Si_(v)B_(w), where M is one or more elements from amongthe group Nb, W, Ta, Zr, Hf, Ti, Mo and x+y+z+v+w=100%, where: Fe x =100% − y − z − v − w Cu y = 0.5 − 2 at % M z = 1 − 5 at % Si v = 6.5 −18 at % B w = 5 − 14 at %

 where v+w>18 at %.
 7. Low-pass filter as claimed in claim 6,characterized in that the following relations hold: Fe x = 100% − y − z− v − w Cu y = 1 at % M z = 2 − 3 at % Si v = 14 − 17 at % B w = 5 − 14at %

 where v+w=22-24 at %.
 8. Low-pass filter as claimed in claim 1,characterized in that the alloy has the compositionFe_(x)Zr_(y)Nb_(z)B_(v)Cu_(w), where x+y+z+v+w=100 at %, where: Fe x =100 at % − y − z − v − w Zr v = 2 − 5 at % Nb z = 2 − 5 at % B v = 5 − 9at % Cu w = 0.5 − 1.5 at %

 where y+z>5 at % and y+z+v>11.
 9. Low-pass filter as claimed in claim8, characterized in that the following relations hold: Fe x = 83 − 86 at% Zr y = 3 − 4 at % Nb z = 2 − 5 at % B v = 5 − 9 at % Cu w = 1 at %

 where x+z>7 at %, and y+z+v is 12-16 at %.
 10. Low-pass filter asclaimed in claim 1, characterized in that the alloy has the compositionFe_(x)M_(y)B_(z)Cu_(w), where M is an element from among the group Zr,Hf, Nb and x+y+z+w=100 at %, where: Fe x = 100 at % − y − z − w M y = 6− 8 at % B z = 3 − 9 at % Cu w = 0 − 1.5 at %.


11. Low-pass filter as claimed in claim 10, characterized in that thefollowing relations hold: Fe x = 83 − 90 at % M y = 7 at % B z = 3 − 9at % Cu w = 0 − 1.5 at %.


12. Low-pass filter as claimed in claim 1, characterized in that thealloys have the composition (Fe_(0.98)Co_(0.02))_(90−x)Zr₇B_(2+x)Cu₁,where x=0-3, wherein Co may be replaced by Ni with a correspondingadjustment of the remaining alloy components.
 13. Low-pass filter asclaimed in claim 12, characterized in that x=0.
 14. Low-pass filter asclaimed in claim 1, characterized in that the alloy has the compositionNi_(x)Fe_(y)M_(z), where M is one or more elements from among the groupNb, Mo, Mn, Si, Al, Ti, Cr and x+y+z=100 at %, where Ni x = 35 − 68 at %Fe y = 100 at % − x − z M z = 0 − 9 at %.


15. Low-pass filter as claimed in claim 14, characterized in thatx=50-65 at % and z=0.5-3 at %.
 16. Low-pass filter for a diplexer, thefilter comprising means for separating low-frequency signals of analogcommunications systems from high-frequency signals of digitalcommunications systems, the separating means comprising a plurality ofseries-connected inductive chokes, each choke.having a magnetic coremade of a toroidal tape of an iron-containing alloy and an impedencewhich increases with increasing frequency and suppresses harmonics ofthe low-frequency signals.
 17. Low-pass filter for a diplexer forseparating low frequency signals of analog communications systems fromhigh frequency signals of digital communications systems, the low-passfilter comprising a plurality of longitudinal inductances connected inseries, such longitudinal inductances (i) comprising magnetic cores madeof an alloy and (ii) having impedence which increases with increasingfrequency and suppresses harmonics of the low-frequency signals.